1. Field of the Invention
The present invention relates to an illumination optical system which illuminates an object of irradiation, an aligner and an exposure method including the illumination optical system, and used in lithography for manufacturing various kinds of devises such as semiconductor devices and liquid crystal display devices, and a designing method of a device.
2. Description of the Related Art
With high densification of semiconductor elements, microfabrication of patterns formed with lithography has been advanced. As a method for accurately forming micropatterns, a multiple exposure process using an alternating phase shift mask is proposed.
The method of the multiple exposure process will be described by using FIGS. 21A to 21C.
For example, when a gate layer in a logic circuit of a semiconductor device is to be formed, an element isolation region 101 and an active region 102 defined by the element isolation region 101 exist on a silicon substrate as shown in FIG. 21A. The gate layer is ordinarily formed to be across the active region 102.
On forming the gate layer, a first mask pattern 104 is firstly exposed onto a photoresist (not shown) of the silicon substrate by using a first photomask 103 that is an ordinary chrome mask, an attenuated phase shift mask or the like. Thereafter, as shown in FIG. 21B, a second mask pattern 106 is exposed onto the photoresist to overlap the first mask pattern 104 (double exposure) by using a second photomask 105 that is an alternating phase shift mask.
The alternating phase shift mask is constructed so that the phases of adjacent mask patterns are shifted by π (180°). When exposure is performed by using the alternating phase shift mask, the light intensity becomes very steep, and by using a relatively small illumination system, very wide depth of focus can be obtained. As a result, as shown in FIG. 21C, a gate layer 111 which is narrowed in width only on the active region 102 is formed. When a gate layer is formed with double (multiple) exposure like this, an extremely wide exposure margin can be obtained as compared with single exposure.
An alternating phase shift mask makes desired microfabrication possible with an extremely wide margin as described above, but on the other hand, it has the problems that its manufacturing process is complicated and manufacture cost is high. The problem of the three-dimensional structure of the alternating phase shift mask cannot be treated lightly. On manufacturing an alternating phase shift mask, the lower portion of a light shield film between 0 and π is in the shape of eaves. This is made to have a three-dimensional structure for the purpose of eliminating unbalance of intensity of light which is transmitted through each open portion. However, if miniaturization of processing dimensions advances, and the light shield film becomes small, this is expected to be a large problem in manufacturing photomasks.
In this respect, an art of Japanese Patent Application Laid-open No. 2001-126983 is proposed as a multiple exposure process. This art is the art of dividing mask data into two, and transferring the divided patterns by using optimum illumination systems and optical conditions for the respective patterns and using a double pole illumination as at least one illumination system. This is the multiple exposure process without using an alternating phase shift mask, and is advantageous in the respect that the manufacture cost is low. However, this method uses one photomask for one pattern on exposing a number of patterns, and therefore, it has the problem of being incapable of obtaining a sufficient manufacture process margin.
Further, with miniaturization of processing dimensions, the need for patterning so that micropatterns differing in pitch exist in close vicinity to one another has been intensified recently, and an exposure process sufficiently corresponding to such a construction has been demanded. As a method capable of corresponding to such a demand, the above described multiple exposure process is also one of potential methods, but it is difficult for the multiple exposure process to obtain a sufficient manufacture process margin as described above.